There are two main approaches for beam steering known in the prior art. The first is to use a single beam propagating in some predetermined direction and then this “ready to use” beam is steered with a movable reflector such as a mirror. Any optical beam can be used as such a single beam, but a more powerful beam can be developed by combining of the beams of a laser array as shown in FIG. 1.
In this approach, the final combined beam has a fixed direction before steering; therefore, an array of low-divergent (essentially collimated) beams can be used as shown in FIG. 1 (only three laser beams from array are shown for simplicity of illustration).
The relative phases of the coherent beams are controlled to obtain constructive interference of all the beams in a chosen direction X in FIG. 1 (this is called “coherent beam combining” where all of the combined beams have the same wavelength) or all the beams are combined in the same direction by dichroic mirrors, prisms, gratings, etc. (this is called “spectral beam combining” where the combined beams have different wavelengths). See, for example, T. Y. Fan, “Laser Beam Combining for High-Power, High-Radiance Sources”, IEEE J. of Selected Topics in Quan. Elect., Vol. 11, No. 3, May/June 2005. Spectral beam combining does not require phase measurement and control and therefore it is not further discussed here.
Coherent beam combining involves saving of combined beam direction by phase control—or more accurately—by phase locking of all the beams from a laser array which means that these beams have to have fixed phases providing the fixed chosen direction of main interference lobe of combined beam. In this case, the steering of this lobe or beam steering is accomplished by a movable reflector such as a mirror shown in FIG. 1. To implement this method, small portion of the beam intensities is usually taken by plane beam sampler (FIG. 1) for beam phase locking by a dithering technique (see, T. R. O'Meara, “The multidither principle in adaptive optics,” J. Opt. Soc. Am. 67, 306, 1977) or by stochastic parallel gradient descent feedback technique (see, L. Liu, M. A. Vorontsov. “Phase-locking of tiled fiber array using SPGD feedback controller,” Proc. SPIE, 5895, 58950P-1, 2005). These techniques provide for development of far field image of interfering beams (usually by lenses), measurements of beam intensities at dithering frequencies or combined beam intensity depending on technique, and beam phase controls through feedbacks. Since the combined beam has to be developed in a fixed direction X, the sampled portion of beam has fixed direction also (FIG. 1). This portion can be taken with a low-reflecting beam sampler or even as a reflection from some bright spot on a target (see the T. R. O'Meara article). However, the described method of beam steering with movable reflectors does not allow agile beam steering because it requires motion.
To overcome this obstacle, another approach shown in FIGS. 2a and 2b, can be used (again, only three laser beams from array are shown for simplicity). In this case, an array of rather wide-divergent beams is used and beam steering is accomplished by constructive interference of multiple coherent laser beams as a main lobe in a desired direction. This can be done through the proper modification of beam phases and involves beam phase measurements, calculations of phase relations between the beams for required constructive interference of the main lobe direction, and control of beam phases through feedback.
Actually, the angles of beam divergences θ will define the maximum angle of beam steering which is ˜±θ/2). If all the beams from the laser array have the same phase, the main interference lobe of a randomly positioned laser array is directed normal to the surface of the array output as shown in FIG. 2a. In the case of orderly positioned beams, the intensities of sidelobes may have intensities comparable with the intensity of main lobe; therefore, such a design is not really practical. To direct the main lobe of the combined beam in some selected direction (see FIG. 2b), it is necessary to calculate and apply proper phase shifts between all the beams of the laser array. Such phase shifts will develop a powerful beam in one selected direction (the main lobe) with multiple, suitably low-intensity sidelobes in other directions, which sidelobes look like much like noise. More beams positioned closer to each other in the laser array will provide a higher intensity of the combined beam (the main lobe) relative to the noise-like sidelobes. The advantage of this technique is that it is not necessary to move or reflect mechanically any of the beams of the laser array for beam steering; they have absolutely stable positions in space. Electronic control of the phases, which can be implemented for quick action, is required for beam steering in the apparatus of FIG. 2b. So this electronic phase control technique provides faster beam steering than the mechanical approach of FIG. 1.
For phase measurements, the spatially stable sample of beam interaction area (or even some reflection from a bright spot on a target—see the T. R. O'Meara article mentioned above—can be used; however, this sample has to be fixed in the space for measurements, which is not possible in the case of a steering beam) or image of laser array output has to be formed. Prior art optical systems consisting of a plane beam sampler and imaging optics used for collimated beam combining as shown in FIG. 1. For beam steering approach shown in FIG. 2a, such systems start being rather cumbersome for large angles of steering and require the application of special high numerical aperture optics as shown in FIG. 2a. Simple geometrical considerations show that in the cases of steering angles close to ±45° or larger, sampling every single beam from the laser array as a whole cannot be taken by a plane sampler because of the very large beam divergence, which should be close to 90° or larger. Thus, if the steering angle is close to or more than ±45°, the optical systems consisting of plane reflectors/samplers and standard optics cannot be used to take a sample of the beam array as a whole to make its stable image.
The present disclosure describes a new design of a relatively compact imaging system which can be applied for very large angles of steering close to ±90° and provides stable spatial positions of beam output images. Simultaneously, the disclosed systems can provide a protective cover for laser array to help protect against environmental impacts.
The prior art also includes:    Kenneth Li, “Etendue efficient coupling of light using dual paraboloid reflectors for projection displays,” Proc. of SPIE, 4657, 1 (2002).    T. R. O'Meara, “The multidither principle in adaptive optics,” J. Opt. Soc. Am. 67, 306, (1977).    T. R. O'Meara, “Stability of an N-loop ensemble-reference phase control system,” J. Opt. Soc. Am. 67, 315, (1977).    L. Liu, D. N. Loizos, P. P. Sotiriadis, and M. A. Vorontsov, “Coherent combining of multiple beams with multidithering technique: 100 kHz closed-loop compensation demonstration,” Proc. of SPIE, 6708, 67080D-1, (2007).    S. D. Lau, J. P. Donnelly, C. A. Wang, R. B. Goodman, and R. H. Rediker, “Optical phase difference measurement and correction using AlGaAs integrated guided-wave components,” IEEE Photon. Technol. Lett., 3, 902, (1991).    B. Golubovic, J. P. Donnelly, C. A. Wang, W. D. Goodhue, R. H. Rediker, “Basic module for an integrated optical phase difference measurement and correction system,” IEEE Photon. Technol. Lett., 7, 649, (1995).